In a video disc presently in use for a video disc player, video and audio information is stored in a multiplex form consisting of a video signal carrier which is frequency modulated with a standard NTSC (National Television System Committee of U.S.A.) video signal and two-channel audio signal carriers each of which is frequency modulated with an audio signal. The video signal carrier is higher in frequency than the audio signal carriers and is superposed in the form of pulsewidth modulation on the audio signal carriers in the multiplex signal recorded on the video disc.
In the case of a video disc of the optically scanned type, the multiplex signal thus consisting of the frequency modulated video and audio signal carriers superposed on each other is stored as a series of depressed areas or "pits" formed in each or one information-carrying face of the video disc and arranged in a spiral track or a number of concentric tracks about the center axis of the disc. The video and audio information thus stored is read out by optically scanning the individual pits along the spiral track or each of the concentric tracks by means of a beam of laser light and thereby detecting the lengths of and spacings between the scanned pits. During playback of such a video disc, the disc is driven for rotation about the center axis thereof usually at a fixed velocity and the beam of the laser light is displaced radially of the disc by a tracking device forming part of the video disc player. The laser beam directed to the target track is reflected from the information-carrying face of the video disc or passed through the disc. The beam of light thus reflected from or passed through the video disc is converted into an electric reproducing signal by means of a photoelectric transducer unit for further conversion into video and audio signals.
As the beam of the laser light scanning the target track of the video disc is displaced radially of the information-carrying face of the disc, the frequency characteristic of the reproducing signal picked up from the video disc varies and the signal levels of high frequency components of the reproducing signal attenuate more in a central area than in an outer peripheral area of the disc. In order to compensate for such attenuation of the signal level, it has been proposed to provide signal correcting means in the reproduction network of an optical video disc player. The signal correcting means of this nature comprises an equalizer circuit adapted to produce an increased gain in the reproducing signal in response to the attenuated frequency components of the reproducing signal supplied to the equailizer network.
The signal level of the reproducing signal read out from an optically scanned video disc also unwantedly varies with the fluctuations in reflection rate of the scanned areas of the disc and the variation in the radiation power of the laser beam. The equalizer circuit in the known signal correcting means is not capable of compensating for such variation in the level of the reproducing signal and, for this reason, the attenuated high frequency components of the reproducing signal can not be corrected satisfactorily.
In the case of a video disc player of the capacitance scanning type, such unwanted variation in amplitude of the reproducing signal read out from the video disc may occur due to presence of small dusts on the surface of the video disc or fluctuations in dielectric constant along the tracks of the video disc.
A first important object of the present invention is, therefore, to provide a video-disc information reading and reproducing apparatus featuring an improved reproducing signal correcting device capable of properly compensating for the attenuation in the signal levels of high frequency components of the reproducing signal read out from a video disc without respect to variations in amplitude of the reproducing signal due to the fluctuations in the reflection rate or dielectric constant of the scanned areas of the disc, the variations in the radiation power of the laser beam, the presence of small dusts on the disc.
On the other hand, it sometimes happens that an optically scanned video disc has information recorded on only one face thereof. When such a video disc is put to use for playback purposes in a video disc player without knowing that the face of the video disc being scanned carries no recorded information, not only the information pickup system of the video disc player can not read out information but the various servo systems incorporated in the video disc player are held inoperative in the absence of the control signals which would be otherwise supplied to the servo systems. These servo systems include spindle and tangential servo systems to control the rotation of a video disc and tracking and focus servo system for controlling the path of the scanning laser beam directed at the target track of the video disc. When such servo systems are held inoperative with the video disc being driven for rotation, there would arise a danger that the mechanical driven member or members of any of the servo systems might be initiated into motion in an uncontrolled condition.
It is, for this reason, of especial importance to detect the presence or absence of recorded information on the face of the video disc being scanned. Such detection could be performed by the use of the output signals produced by the dropout detection circuit incorporated in an ordinary optical video disc player. Satisfactory results can not however be achieved by such an expedient because of the face that detecting dropouts of reproduced information is essentially different in effect from the detection of the presence or absence of recorded information on a video disc.
It is, accordingly, a second important object of the present invention to provide a video-disc information reading and reproducing apparatus including, in addition to the above mentioned reproducing signal correcting device, a device for detecting the presence or absence of recorded information on the face of a video disc.
A pick-up device of the video disc player is moved radially of the information-carrying face of the disc by the aid of the tracking servo system under the control of a tracking error signal indicative of the location of the reading or detecting point of the pick-up device with respect to the target track on the disc. The reading or detecting point corresponds to a scanning spot in the case of the optical video disc player. To control the tracking servo system reliably on the basis of such an error signal, it is important that the servo loop of the tracking servo system be closed when or after the detecting point is moved close to the target track on the video disc. If, in the case of the optical video disc player, the servo loop is closed and the servo system is locked in before the scanning spot of light is moved close to the target track, it may happen that the tracking mirror forming part of the optical pick-up system and operative to deflect the scanning beams in a radial direction of the video disc is abruptly initiated into motion to reach the target track and thus overshoots the target track. In an extreme case, the tracking mirror may be caused to oscillate and disable the tracking servo system form being locked in. Such an event may be caused not only during scanning of a video disc but generally when the servo loop of the tracking servo system is to be closed from an open condition.
It is, therefore, a third important object of the present invention to provide a video disc information reading and reproducing apparatus including an improved tracking servo system which can be locked in reliably and in a stable condition when the servo loop of the system is to be closed from an open condition.